Tissue engineering is generally defined as the creation of tissue or organ equivalents by seeding of cells onto or into a scaffold suitable for implantation. The scaffolds must be biocompatible and cells must be able to attach and proliferate on the scaffolds in order for them to form tissue or organ equivalents. These scaffolds may therefore be considered as substrates for cell growth either in vitro or in vivo.
The attributes of an ideal biocompatible scaffold would include the ability to support cell growth either in vitro or in vivo, the ability to support the growth of a wide variety of cell types or lineages, the ability to be endowed with varying degrees of flexibility or rigidity required, the ability to have varying degrees of biodegradability, the ability to be introduced into the intended site in vivo without provoking secondary damage, and the ability to serve as a vehicle or reservoir for delivery of drugs, cells and/or bioactive substances to the desired site of action.
A number of different scaffold materials have been utilized, for guided tissue regeneration and/or as biocompatible surfaces. Biodegradable polymeric materials are preferred in many cases since the scaffold degrades over time and eventually the cell-scaffold structure is replaced entirely by the cells. Among the many candidates that may serve as useful scaffolds claimed to support tissue growth or regeneration, are included gels, foams, sheets, and numerous porous particulate structures of different forms and shapes.
Among the manifold natural polymers which have been disclosed to be useful for tissue engineering or culture, one can enumerate various constituents of the extracellular matrix including fibronectin, various types of collagen, and laminin, as well as keratin, fibrin and fibrinogen, hyaluronic acid, heparin sulfate, chondroitin sulfate and others.
Other common polymers that were used include poly(lactide-co-glycolide) (PLG). PLG are hydrolytically degradable polymers that are FDA approved for use in the body and mechanically strong (Thomson R C, Yaszemski M J, Powers J M, Mikos A G. Fabrication of biodegradable polymer scaffolds to engineer trabecular bone. J Biomater Sci Polym Ed. 1995; 7(1):23-38; Wong W H. Mooney D J. Synthesis and properties of biodegradable polymers used as synthetic matrices for tissue engineering. In: Atala A, Mooney D J, editors; Langer R, Vacanti J P, associate editors. Synthetic biodegradable polymer scaffolds. Boston: Birkhäuser: 1997. p. 51-82). However, they are hydrophobic and typically processed under relatively severe conditions, which make factor incorporation and entrapment of viable cells potentially a challenge.
As an alternative, a variety of hydrogels, a class of highly hydrated polymer materials (water content higher than 30% by weight), have been used as scaffold materials. They are composed of hydrophilic polymer chains, which are either synthetic or natural in origin. The structural integrity of hydrogels depends on cross-links formed between polymer chains via various chemical bonds and physical interactions. Hydrogels used in these applications are typically degradable, can be processed under relatively mild conditions, have mechanical and structural properties similar to many tissues and the extracellular matrix, and can be delivered in a minimally invasive manner (Lee K Y, Mooney D J. Hydrogels for tissue engineering. Chem Rev. 2001 July; 101(7):1869-79). Various polymers have therefore been used to process hydrogels. For example, those polymers include collagen, gelatin, hyaluronic acid (HA), and chitosan.
Use of natural polysaccharides represents also a promising alternative for making scaffolds based on hydrogels, because they are non antigenic and non immunogenic, and some of them present antithrombotic effects and interactions with vascular growth factors. Furthermore, due to their plasticity properties, those polysaccharides based hydrogels may be shaped in various forms to allow the design of therapeutic implant or graft biomaterials.
For example, Chaouat et al. (Chaouat M, Le Visage C, Autissier A, Chaubet F, Letourneur D. The evaluation of a small-diameter polysaccharide-based arterial graft in rats. Biomaterials. 2006 November; 27(32):5546-53. Epub 2006 Jul. 20) designed a novel polysaccharide based scaffold prepared by using a mixture of pullulan and dextran. Chemical cross-linking of polysaccharides was carried out using the cross-linking agent trisodium trimetaphosphate (STMP). Thereafter, the effectiveness of an arterial material prepared with this scaffold was demonstrated in vivo.
However, despite the advantageous of using polysaccharides for preparing scaffolds as described in Chaouat et al. (2006), the default of porosity of the resulted scaffold remains a drawback to envisage an effective use for therapeutic purposes. Actually, porosity is an essential feature to allow the proliferation, integration and differentiation of the cells inside the scaffold, so that the material can be used as a cell reservoir to reconstruct in vivo the tissue or organ.
Therefore there is still an existing need in the art to develop a method for preparing porous scaffold matrices that can be used for therapeutic purposes.